1. Field of the Invention
This invention relates to soldering systems of the type that provide localized soldering through the use of lasers. Such structures of this type generally allow flexible. printed circuit boards to be locally soldered to ceramic substrates.
2. Description of the Related Art
It is known in prior, soldering systems for soldering flexible printed circuit boards to substrates to employ a conventional wire bonding technique. In particular, wires of approximately 0.3 mm in diameter are initially bent so they can be easily bonded to the substrate and the circuit board while the circuit board is being held perpendicular to the substrate. The wires are first bonded to the circuit board and then they are bonded to the substrate. It was discovered that the operator must be very careful when placing the circuit board against the substrate because the brittle ceramic substrate could not withstand a high load factor. Also, the size of the wires to be bonded was a limiting factor in that an operator could not bond wires smaller than, typically, 0.3 mm in diameter. Consequently, it would be advantageous if such amounts of complex steps could be reduced while using smaller diameter wires and still avoiding a high load factor on the substrate.
In order to avoid possible damage to the substrate by substrate deformation, lasers began to be used in order to simply solder the leads from the flexible printed circuit board to the substrate. Exemplary of this method is U.S. Pat. No. 4,799,755 which is assigned to the same assignee as the present invention. However, it was soon discovered that while the use of the laser eliminated the downward force on the substrate, this particular laser could only be used on a limited variety of substrates, namely, glass/epoxy substrates. This is an important consideration because there is a current need in medical diagnostic devices, especially ultrasound imaging devices, to develop a two-dimensional array of piezoelectric sensors in order to acquire more patient information from the ultrasound imaging device. In order to acquire this greater amount of information, it was contemplated that ceramic substrates with this two-dimensional array of sensors made from the substrate are needed for construction. The method as set forth in the prior art, particularly, that method as set forth in U.S. Pat. No. 4,799,755 would not be applicable in this case because the epoxy/glass substrate technology cannot be applied to this application. In particular, an epoxy/glass substrate is a heat insulator while a ceramic substrate which has been coated with a metal coating is a heat conductor. When soldering is done to a heat insulator, there is very little concern as to how the heat from the soldering process is going to affect adjacent solder joints that have already been completed. However, when soldering to a heat conductor, there is a great concern as to how the heat from the soldering process is going to affect adjacent solder joints. There is also concern not to damage the thermally sensitive piezoelectric material during the soldering process so the Curie temperatures (150.degree.-400.degree. C.) of piezoelectric materials must be exceeded. Therefore, when dealing with highly heat conductive substrates, the operating parameters of the laser soldering process and the dimensions of the solder pads must be carefully controlled. However, a still further advantageous soldering system, then, would be presented if the soldering could be performed on a large variety of substrates while still employing the highly advantageous results achieved through the use of lasers.
It is apparent from the above that there exists a need in the art for a soldering system which can perform localized soldering and which at least equals the performance characteristics of the known laser soldering systems, particularly those of the highly advantageous type disclosed in U.S. Pat. No. 4,799,755, but which at the same time allows the localized soldering to be performed on a greater variety of substrates, especially metallized ceramic substrates. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.